1. Technical Field
The present disclosure relates to a light-emitting device having a patterned surface.
2. Description of the Related Art
Recently, efforts have been devoted to promote the luminance of the light-emitting diode (LED) in order to implement the device in the lighting domain, and further procure the goal of energy conservation and carbon reduction. There are two major aspects to promote luminance. One is to increase the internal quantum efficiency (IQE) by improving the epitaxy quality to enhance the combination efficiency of electrons and holes. The other is to increase the light extraction efficiency (LEE) that emphasizes on the light which is emitted by the light-emitting layer capable of escaping outside the device, and therefore reducing the light absorbed by the LED structure.
Surface roughening technology is one of the efficient methods to enhance luminance. FIG. 7 shows a known LED 700 having a patterned substrate. LED 700 comprises a growth substrate 701, an epitaxial stack, a first electrode 707, and a second electrode 708. The surface 701a of the growth substrate 701 has a plurality of trapezoid depression for improving the light-extraction efficiency. The epitaxial stack comprises a buffer layer 702 grown on the growth substrate, a non-doped semiconductor layer 703 grown on the buffer layer 702, a first semiconductor layer 704 with first conductivity-type grown on the non-doped semiconductor layer 703, an active layer 705 grown on the first semiconductor layer 704, a second semiconductor layer 706 with second conductivity-type grown on the active layer 705. The first electrode 707 is formed on the exposed first semiconductor layer 704, and the second electrode 708 is formed on the second semiconductor layer 706.
The ratio of the pattern width to the width between patterns of the substrate surface 701a is generally designed to be around 1. Therefore, a considerable portion of the substrate surface 701a is still parallel to the surface of the active layer 705a, and the light emitted from the active layer 705 to the parallel substrate surface is easily reflected back to the epitaxial stack because of total internal reflection (TIR) effect and absorbed by the epitaxial stack to generate heat. It worsens both the light extraction efficiency and the heat dissipation problems. Nevertheless, the pattern is usually formed deeper in order to compensate the light loss due to the parallel (unpatterned) region, but the high aspect ratio of the deeper pattern causes difficulty for subsequently epitaxial growth and adversely affects the epitaxial quality.
Another prior technique for roughen surface is to utilize mechanically polishing method to form a randomly distributed rough patterns on the substrate surface. By this method, it is hard to control the roughened dimension, such as the depth or the width. Moreover, the epitaxial quality is not good by growing an epitaxial layer on the randomly rough surface.